A conventional vapor-compression refrigerator having two evaporators with different evaporation temperatures is described in JP-A-58-33075, for example. This vapor-compression refrigerator includes a first decompression unit for decompressing refrigerant flowing into a first evaporator in which the refrigerant is evaporated at a temperature equal to or higher than a freezing temperature, a second decompression unit for decompressing refrigerant flowing into a second evaporator in which the refrigerant is evaporated at a temperature lower than the freezing temperature, and a switching means for switching a refrigerant circulation. The switching means switches between a first circulation where refrigerant is circulated to the first evaporator and a second circulation where refrigerant is circulated to the second evaporator.
However, in the vapor-compression refrigerator described in JP-A-58-33075, refrigerant such as R134a is used, and an evaporation pressure is about 0.2 Mpa at 0° C. In this state, an average refrigerant density is about 10 Kg/M3 that is extremely small as compared with carbon dioxide. Accordingly, even when a compressor operates to suck refrigerant in a low pressure member downstream of the first evaporator, such as an accumulator and a low-pressure pipe between the accumulator and the compressor, because the refrigerant density is low, a refrigerant amount (mass flow amount) sucked by the compressor is small. Therefore, it is difficult to shorten a necessary time for which a pressure in the low pressure member downstream of the first evaporator is reduced to the pressure in the second evaporator.
Thus, when the switching means switches from the first circulation to the second circulation in the vapor-compression refrigerator, a long time is necessary for reducing refrigerant pressure in the second circulation, and cooling capacity of the first evaporator or the second evaporator cannot be effectively improved.
Further, when the second evaporator is evaporated in a low temperature area lower than the freezing temperature, the refrigerant density decreases, and the refrigerant amount sucked into the compressor decreases. Therefore, it is difficult to reduce the temperature of the second evaporator to −5° C. that is actually used.